In the 1950's a proposal was made by Dwin R Craig and John N Street that a photographic contrast mask should be made using a metachromatic oil use. This material had a photochromic property, in that exposure of it to blue light caused it to turn a brillant blue, and exposure to yellow light turned the blue material colourless. Craig and Street proposed that a body should be made bearing a film of this oil or a layer of micro-encapsulated such oil. The body was to be aligned with a transparency and then illuminated with blue light shone through the transparency. A blue contrast mask would then be formed instantly by the photochromic effect. The photochromic reaction of the metachromic oil dye is extremely fast, and Craig and Street propose that the photographic exposure in the reproduction process and the mask-forming exposure are carried out simultaneously, the photochromic body acting in effect as a shutter the speed of which varies with the optical density of the transparency. A US patent application for this proposal was filed in 1958, an equivalent UK patent specification was published in 1962 as No. 891992 and a US patent issued in 1970 as No. 3510305.
For practical purposes, the Craig and Street proposal appears to be useless. It is a feature of their system that the light used in the photographic exposure is of a wavelength to which the photochromatic oil dye is not reactive. Indeed, it is clear that if yellow light was used for the photographic exposure, it would clear the dye, wiping out the contrast mask. It appears that blue light, while tending to form the masking image, is not absorbed by it, since the image is a brilliant blue which implies that blue light is transmitted through it. This will also make blue light unsuitable for use in the photographic exposure. As a result, any attempt to use the body with metachromatic oil dye as a contrast mask in colour reproduction, will dramatically alter the colours in the reproduction and in addition relatively dark areas which are coloured blue will tend to be masked more densely than relatively pale areas coloured yellow, owing to the colour sensitivity of the metachromatic oil dye, so that the contrast masking effect provided will be wholly incorrect. While it may appear that the process might be of some value in monochrome photographic reproduction, the modern practice of using multigrade paper prevents this, as is explained below.
Different grades of photosensitive paper respond to illumination in different ways, so that to print onto one grade of paper will produce a high contrast print and to print onto another will produce a low contrast print. A photographic craftsman would have to stock paper of all different grades and select the appropriate one for any given printing operation. However, multigrade monochrome paper is so designed that it reacts differently to different wavelengths of light, so that the apparent grade of the paper is determined by the colour of the light used in the photographic exposure. Unfortunately for the Craig and Street proposal, the colours used to control the grade behaviour of modern multigrade paper are blue and yellow, pure light of one colour giving the highest contrast image and pure light of the other colour giving the lowest contrast image.
The patent referred to above acknowledges that the particular matachromic oil dye used is not suitable for colour photography, and propose "Suitable mixtures of photochromic bodies or dyes can be employed to produce spectral changes from a clear phase to a neutral gray transmission, and modified by exposure to obscuring light and clearing light having wavelengths beyond the visible range." However, it appears that many years of research were spent in attempting to develop such a material, first at the National Cash Register Company and then at British American Tobacco Ltd, but eventually this research was abandoned without a suitable material having been found.
In FIG. 8 of the patents referred to above, a rough graph is given of the transmission of radiation through the photochromic body against the wavelength of the radiation. In the description of this Figure it is stated that the spectral transmission of the photochromic body should be substantially neutral in both the obscured and cleared phases throughout the "actinic range" ("actinic range" is apparently used in the patents to refer to wavelengths used in the photographic process). The Figure does show a relatively flat transmission characteristic in the range between the clearing and obscuring wavelengths. However, the Figure also shows greater transmission in the clearing and obscuring wavelengths than in the intermediate wavelengths when the material is "cleared" as well as when it is "obscured". Thus the Figure cannot be presenting factual data about the known metachromic oil dyes disclosed in the patents, since these are said to be colourless when clear. To be colourless when clear requires that the transmission of the obscuring blue light and the clearing yellow light are both equal to the transmission of intermediate wavelengths when the material is clear, in contradiction to the Figure. Thus it appears that this Figure in the patents represents wishful thinking and a statement of desiderata rather than a description of any substance known to Craig and Street.
The effect of the Craig and Street proposal appears to have been to divert attention into an unprofitable area, and thus inhibit the creation of a usable method of contrast masking by use of photochromic material.